Pressure swing adsorption (PSA) processes provide an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbed gas can be an impurity which is removed from the less strongly adsorbed gas which is taken off as product, or, the more strongly adsorbed gas can be the desired product which is separated from the less strongly adsorbed gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feedstream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbed gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In PSA processes, a multi component gas is typically passed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, i.e. the more strongly adsorbed components, while at least one other component passes through, i.e. the less strongly adsorbed components. At a defined time, the passing of feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein the pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure in the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. Such PSA processing is disclosed in U.S. Pat. No. 3,430,418, issued to Wagner, U.S. Pat. No. 3,564,816, issued to Batta, and in U.S. Pat. No. 3,986,849, issued to Fuderer et al., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system.
As noted above the more strongly adsorbed components, i.e., the adsorbate, are removed from the adsorber bed by countercurrently depressurizing the adsorber bed to a desorption pressure. In general, lower desorption pressures are preferred in order to provide more complete removal of the adsorbate during the desorption step. In addition, lower desorption pressures can provide a greater capacity differential between adsorption and desorption conditions and thus increase the capacity of the process. However, very low desorption pressures, i.e., below atmospheric pressure, are not often used because of the technical complexities and cost associated therewith, e.g., vacuum pumping and the like. Additionally, in hydrogen purification it is often necessary to provide the desorption effluent stream, also known as tail gas, at a pressure suitable for feeding into a fuel gas header, e.g., 20-100 psia. Accordingly, when vacuum pumps are employed in PSA processes, the discharge pressure is typically maintained at or higher than the fuel gas pressure. When vacuum pumps are not employed, PSA hydrogen processes typically employ a desorption pressure of greater than or equal to the fuel gas pressure.
European Patent No. 015,413 B1, issued Feb. 9th, 1983, discloses a pressure swing adsorption process which employes a two stage countercurrent desorption step for the removal of absorbed components which can achieve subatmospheric pressure. The patent discloses that the desorption pressures are produced with the aid of jet devices, i.e., ejectors, and that gas or gas mixtures which have pressures higher than the desorption pressure and which are formed in the course of the process are used as drive means, i.e., motive gas, for the jet devices. Examples of the drive means include: the adsorption effluent, the feedstream and the effluent from the first countercurrent desorption step. The patent does not disclose the use of a countercurrent purge step in conjunction with the two countercurrent desorption steps.
U.S. Pat. No. 4,813,980, issued to Sircar, discloses a multi-column pressure swing adsorption process for simultaneous production of ammonia synthesis gas and carbon dioxide from a reformer off gas having hydrogen, nitrogen and carbon dioxide as major components accompanied by minor quantities of methane, carbon monoxide and argon as impurities. The PSA system features two groups of adsorber beds in which CO.sub.2 is adsorbed in the adsorbers of the first group, i.e., the A beds, the essentially CO.sub.2 -free effluent being charged to an adsorber of the second group, i.e., the B beds, for removal of minor impurities while discharging an effluent gas having an H.sub.2 /N.sub.2 content stoichiometric for NH.sub.3 synthesis. The CO.sub.2 recovered from the first group of adsorbers is available at a high purity for reaction with the ammonia product for production of urea. The first group of adsorber beds described in the above identified patent employ a two-stage countercurrent desorption step wherein during the first stage the adsorber bed is depressurized by the discharge of the contained gas to near ambient pressure. During the second desorption step the adsorber bed evacuated to sub-atmospheric level with further removal of the CO.sub.2 rich effluent. There is no countercurrent purge step disclosed with regard to the adsorption cycle used in the first group of adsorbers.
One problem with the two stage desorption steps described in European Patent 015,413 B1 and U.S. Pat. No. 4,813,980, is that no provision is made for a countercurrent purge step to be used in conjunction with the two desorption steps. Thus, the two countercurrent desorption steps are functionally similar to one continuous countercurrent desorption step. Countercurrent purge steps are often employed in pressure swing adsorption processes to further desorb adsorbate from the adsorber bed and additionally remove adsorbate remaining in the void spaces of the adsorber bed after the countercurrent desorption step. PSA adsorption cycles that employ a countercurrent desorption step with a countercurrent purge step typically provide enhanced purity, capacity and recovery as compared to PSA adsorption cycles that do not employ a countercurrent purge step. Above-cited U.S. Pat. Nos. 3,430,418 and 3,986,849, for example, disclose the use of countercurrent purge steps in PSA adsorption cycles. In fact, even above-cited U.S. Pat. No. 4,813,980, which discloses two countercurrent desorption steps in the A beds, also discloses the use of a countercurrent purge step but only in conjunction with the single stage countercurrent desorption step in the B beds. As noted above, there is no disclosure of the use of a countercurrent purge step in conjunction with the two stage desorption steps employed in the first group of adsorber beds, i.e., the A beds.
U.S. Pat. Nos. 4,261,716 and 4,331,456, issued to Schwartz et al., are directed to improved processes for recovering hydrocarbons from an air-hydrocarbon mixture, such as the mixture of air and vaporized light hydrocarbon compounds expelled as a result of loading gasoline or the like into storage tanks and tank trucks. The patents disclose an adsorption cycle wherein the regeneration of the adsorber beds is accomplished by evacuating the beds with a vacuum pumping whereby a major portion of the hydrocarbons are desorbed therefrom, subsequently introducing a small quantity of heated hydrocarbon free air into the beds whereby additional hydrocarbons are stripped therefrom and then subjecting the bed to further evacuation by ejector jet pumping while continuing to evacuate the bed by vacuum pumping whereby yet additional hydrocarbons are desorbed therefrom. The air-hydrocarbon mixture produced in the regeneration of the beds is contacted with a liquid adsorbent whereby a major portion of the hydrocarbons are desorbed therefrom and recovered. The non-absorbed gas from the liquid absorbent is used as the motive gas for the ejector. Thus, the processes described in U.S. Pat. Nos. 4,261,716 and 4,331,456 require that both the first countercurrent depressurization step and the purging step be conducted under vacuum conditions. Further, the patents require that the purge gas be heated.
Thus, improved pressure swing adsorption processes are sought which employ multiple desorption steps in addition to a countercurrent purge step wherein the first countercurrent depressurization step can be conducted at above-atmospheric pressure. Improved processes are further sought wherein the countercurrent purge step is conducted at or near the adsorption temperature and further that the tail gas obtained from the process be available at a tail gas pressure that is sufficient for use as a fuel gas.